1. Field of the Invention
The present invention relates a method and apparatus for cooling and quenching metallic work pieces. In particular, the invention relates to a method and apparatus for cooling heat-treated parts through an air quenching system especially adapted for use in cooling parts having complex shapes, such as various components used in jet and gas turbine engines. The method and apparatus disclosed below are designed primarily for the purpose of uniformly cooling complex-shaped parts that under convention quenching techniques exhibit varying cooling rates. The method and apparatus disclosed below may be adapted to produce controlled differential cooling rates at different portions of the part.
2. Background of the Related Art
Metal parts are commonly heat-treated to improve the wear and strength characteristics of the part. The heat-treating of steel and other metals is a well known, but highly complex process that is designed to alter the microstructure of the material. Strength and wear characteristics of a particular type of steel are normally dependant upon the percentage of carbon and other alloy materials that make up the steel, and also upon the rate that the part is cooled after it has been heated. It is common to cool heated parts by immersing the part in a fluid bath. This process of cooling the part is referred to in the trade as xe2x80x9cquenchingxe2x80x9d.
Heat-treating refers to the heating of a steel part, usually in a furnace, to a temperature above a critical temperature whereupon the steel undergoes a phase transformation. Quenching refers to the process of rapidly cooling the heated part at a cooling rate that is sufficient to maintain certain molecular compositions of the metal acquired during heating, or to obtain certain desired molecular characteristics that form during the quenching process. As a general proposition, quenching of steel work pieces has been conventionally accomplished by immersing the part in a liquid coolant, typically water or oil.
In the heat treatment of metals, a wide variety of cooling arrangements have been utilized in an effort to achieve a uniform cooling of the work piece. For most applications, uniform cooling of the entire work piece is desired because that will promote the development of a uniform grain structure within the metal composition and minimize distortion of the piece. Various cooling methods have been employed in an effort to develop a desired microstructure of the material and desired mechanical properties while avoiding physical defects in the part, such as cracking or distortion of the part. It is also desirable to also to control residual stresses within the part, which can affect the machinability of the part during subsequent manufacturing steps and also affect the operating life and characteristics of the part.
Certain parts are subjected to extremely high stresses during use. For example, various components in jet aircraft engines and gas turbine generators, particularly the rotational components, are subjected to very high centrifugal forces and thermal stress during use. Such parts also typically have very complex shapes, with a portion of the part being relatively thick and thus having a relatively large mass, while other portions of the part are quite thin and have a relatively low mass. When heated, the thick, massive portions of the part naturally retain a large amount of heat energy. Because heat dissipates quite quickly from the thin portions of the part but is retained for a longer period of time in the more massive portions of the part, it is extremely difficult to cool such complex-shaped parts uniformly.
Quenching has commonly been performed with water, oil and other liquid coolants. For parts having complex shapes, though, the use of a liquid coolant does not ordinarily provide uniform cooling throughout the part. A liquid coolant will cool the surface of the part very rapidly. However, the inner portion of the thicker and more massive portion of the part cools at a much slower rate. The difference in the cooling rates between the surface of the part and the inner portions of the part result in the creation of internal stresses in the part. Such internal stresses can cause substantial distortion of the part, particularly during later machining and use. Jet and gas turbine engine parts must ordinarily be manufactured to very tight tolerances, and so the amount of permissible distortion during machining is very small.
While an oil bath is the most common quenching medium used for heat-treating purposes, air and other cooling gases have also been used in certain limited circumstances to cool heated parts. Air quenching has the advantage of producing a slower cooling of the part than can be achieved with an oil bath. A variety of methods and apparatus for cooling work pieces with air are known. However, these known methods have in most instances only a limited capability to cool of work pieces of relatively simply geometries. For example, U.S. Pat. No. 2,305,811 to Oeckl relates to the heat treatment of light metal work pieces. The work piece is contained within a chamber, and cooling fluid is supplied through nozzles in the walls. The work piece is subjected to a cloud of atomized cooling fluid, which is then exhausted from the chamber. As another example, U.S. Pat. No. 4,278,421 to Limque et al. discloses an industrial furnace that includes a means for supplying a quenching gas. The quenching gas is circulated by a heavy-duty blower that directs air to a funnel-shaped hood for delivering the air to the work piece for cooling. U.S. Pat. No. 4,769,092 to Peichl et al. discloses the use of nozzles for spray arms for directing a cooling medium onto a work piece.
Statutory Invention Registration No. H777 to Natarajan discloses a method for quenching metal work pieces by directing streams of gas coolant at high velocity and flow rates against the work piece. U.S. Pat. No. 5,770,146 to Ebner relates to a stream for the heat treatment of metallic parts that includes a number of tubular nozzles for directing a cooling medium against the part. The nozzles include telescopically retractable extensions for adjusting the distance between the nozzle and the part. U.S. Pat. No. 6,074,599 to Murty et al. relates to an air quenching system that includes a plurality of air discharge orifices, and a corresponding plurality of air exhaust orifices for circulating air through a cooling chamber. Parts are transported through the cooling chamber on an air previous conveyor belt so that the parts can be cooled from cooling air supplied from both above and below the conveyor.
Additional quenching and cooling systems are disclosed in U.S. Pat. No. 3,470,624 to Plotkowiak, U.S. Pat. No. 610,435 to Pfau et al., U.S. Pat. No. 4,653,732 to Wunning, U.S. Pat. No. 4,767,473 to Berg, U.S. Pat. No. 4,810,311 to Economopoulos, U.S. Pat. No. 4,938,460 to Wechselberger et al., U.S. Pat. No. 2,890,975 to Lenz, U.S. Pat. No. 5,419,792 to King et al.
However, the uniform cooling of work pieces having a complex size and shape requires a different cooling method and apparatus than heretofore has been disclosed or reported. As mentioned, such parts, particularly rotational parts for jet engines, have varying thickness and commonly have protrusions that impede or block the flow of cooling fluid. Consequently, an improved method and apparatus for cooling and quenching particularly rotational parts having complex shapes and cross sections is desired.
A method and apparatus for cooling and quenching heat-treated metallic work pieces is disclosed. The invention is especially adapted for use in quenching work pieces that are later machined and used as components in jet and gas turbine engines. The work piece is typically round or circular in shape. Consequently, it has a radial cross-section that is uniform about its entire circumference. Additionally, the radial cross-section of the part, when viewed from the axis to the outer circumference of the part, has a complex geometry that includes at least one portion that is relatively thick and has a large mass, and at least one portion that is relatively thin and has a low mass.
The apparatus includes an appropriate fixture for supporting the work piece, preferably in a horizontal orientation. Specified portions of the work piece are surrounded by a set of tubes used for directing a compressed air quench onto the work piece for cooling. At least one, and preferably several tubes are located above the work piece, and at least one and preferably several tubes are located below the work piece. The tubes are likewise circular in shape, and preferably oriented horizontally on the fixture. The work piece is placed onto the fixture so that it shares a common axis with the air quench tubes. The air quench tubes are placed in close proximity to the relatively thicker and more massive portion of the work piece.
Each tube is connected to a source of compressed air for supplying the air quench to the work piece. Additionally, each tube is provided with a multiplicity of bores around the circumference of the tube. The bores, which are essentially holes drilled into the tube, are aimed at the work piece so that the compressed air flows onto the thick, massive portion of the work piece, and away from the thin, less massive portion of the work piece.
The fixture is designed to include a number of air quenched delivery tubes in a fixed location underneath the work piece. Additionally, the fixture includes several tubes that are mounted on a slide for moving the tubes toward and away from the work piece as needed.
The apparatus also optionally includes shielding that essentially blocks the flow of compressed air and redirects it away from the thin portions of the part.
The method of the invention includes the steps of heat treating a work piece as described above to a pre-determined temperature and for a pre-determined time; removing the work piece from the furnace and placing it into a fixture for cooling; placing a set of circular tubes in close proximity to the thicker, more massive portions of the work piece; providing a source of compressed air to the tubes; and, directing the compressed air onto the part so that most of the cooling air is directed onto the thicker, more massive portions of the part and that less cooling is directed onto the thinner, less massive portions of the part.
With this apparatus and method, the cooling rate of the thicker, more massive portions of the part is maximized, while the cooling rate of the thinner, less massive portions of the part is simultaneously minimized. The invention disclosed herein thereby provides an overall cooling of the entire work piece at a much more uniform rate than is possible with conventional quenching methods. By cooling both the thicker more massive portions of the part at nearly the same rate as the thinner less massive portions of the part, the wide variation of internal stresses that are normally produced during conventional quenching is avoided. As a result, the part may be machined more uniformly, with little to no deformation as compared to parts that have been quenched using conventional methods.
The method and apparatus may be further adapted to produce controlled differential cooling rates upon different portions of the part. In other words, it is possible to manipulate the method and apparatus disclosed herein to invert the natural cooling rates of the part so that the more massive portions of the part actually cool more quickly than the thin portions of the part, and thereby selectively produce a certain desired internal stress in the part in order to achieve a certain desired characteristic.
Other objects and advantages of the present invention will become apparent from the following description, which sets forth by way of illustration and example certain preferred embodiments of the present invention.